Hydraulic Lash Adjusters (HLA) for taking up slack in valve trains are well known. One common type of valve train comprises a rocker arm mounted for pivotal movement about a central rocker shaft. One end of the rocker arm comprises a roller mounted on an axle carried by the rocker arm. The roller is for engaging a cam mounted on a cam shaft. The other end of the rocker arm carries a hydraulic lash adjustor having a ball end which engages a socket of a stem of a valve for an engine cylinder. The cam has a base circle and a lift profile (i.e. a lobe) and as the cam shaft rotates, when the lobe engages the roller the rocker arm pivots about the central shaft and the HLA exerts a force on the valve stem depressing the valve stem against the force of a valve spring and thus opening the valve. As the peak of the lift profile passes out of engagement with the roller, the return spring begins to close the valve. When the base circle again comes into engagement with the roller, the valve is closed.
As is well known, a typical HLA comprises an oil-containing chamber defined between an outer body and a plunger assembly slideably mounted within the outer body, and a spring arranged to enlarge the chamber by pushing the plunger assembly outwardly from the outer body to extend the HLA. Oil flows into the chamber via a one way valve, but can escape the chamber only slowly, for example, via closely spaced leak down surfaces. Accordingly, a HLA can extend to accommodate any slack in the valve train assembly, such as between the cam and the roller but, after it is extended, the incompressible oil in the chamber provides sufficient rigid support for the HLA to open the valve when the rocker arm pivots (i.e. it prevents the plunger assembly being pushed back inwardly of the outer body so that the HLA acts as a solid body). Typically, the HLA has a second chamber, defined by the plunger assembly, on the other side of the one way valve from the first chamber and which is in fluid communication with the engine's oil supply. Oil supplied from the engine's oil supply is retained within the second chamber and flows into the first chamber through the one way valve when the HLA extends.
It is important that air trapped in the second chamber above the level of oil in that chamber can be purged from the second chamber when the oil level rises. To that end, some hydraulic lash adjusters are provided with a very small diameter aperture that opens into the second chamber and that allows air to purge from the chamber when the oil level rises. The diameters of these holes are large enough to allow sufficient air to purge from the system but not so large as to allow un-desirable oil leakage.
JP 3217604 describes a system in which a float is provided on the surface of the oil in a HLA chamber and which rises as the oil rises in the chamber and which blocks an air purge aperture when the oil completely fills the chamber to prevent oil leaking from the chamber.